Self-lubricating composite friction part

ABSTRACT

Disclosed is a self-lubricating composite friction part ( 1 ) that can be subjected, during operation, to temperatures that are at least equal to 250° C. The part includes, along the friction surface ( 2 ), a single layer of a material consisting of weft and warp yarns made of polytetrafluoroethylene, the material being impregnated with a thermostable resin having a glass transition temperature that is at least equal to 250° C. It is applied to a reinforcing layer ( 3 ).

The invention relates to a self-lubricating composite friction partintended for applications without utilizing lubricant between this partand an opposing part, therefore having a low coefficient of frictionwith the latter, and/or involving temperatures greater than 250° C. andcapable of ranging up to 300° C., or even peaking at 320° C. Such afriction part can be in particular a joint or a slide.

In order to satisfy this type of constraint, it has already beenproposed to cover the surface of the mechanical part with liningsimpregnated with a resin forming a matrix, but these coverings do notmake it possible to obtain a low coefficient of friction combined with agood performance at high temperatures.

Thus, document GB-1 439 030 describes a friction covering, in particularfor a bearing, comprising a friction layer formed of a weaving ofadjacent cords formed by strands with a low coefficient of frictioncontaining a fluorocarbon-containing resin; the surface of these cordshas irregularities, with raised and depressed portions, and the strandsand the cords are embedded in a plastic material. The strands are formedby fibres made of a material such as PTFE, which does not chemicallybond to any plastic material; these fibres are however anchored in theabovementioned covering; the PTFE fibres can be mixed with strands ofcotton. In the example described, the fabric extends helically beingbordered by a helical assembly of glass fibres; the assembly is embeddedin an epoxy or polyester resin. The layer formed by glass fibres isthicker than the layer formed by PTFE strands. In particular due to thenature of the resin used as matrix, it is understood that such acovering when in operation can barely withstand temperatures of 200° C.

Also, it has been envisaged, in document JP-H0425669, to activate thesurface of PTFE fibres so as to secure the fibres within a matrix inwhich these fibres are mixed at a concentration which is barely 5% atmost. Such a configuration moreover does not make it possible to obtaina good mechanical performance, at high temperature, with a lowcoefficient of friction.

Repeating certain teachings from document U.S. Pat. No. 2,804,886,document U.S. Pat. No. 3,804,479 proposes another type of friction layerwhich contains filaments of Teflon® and adherent filaments of Dacron®which are woven sufficiently loosely to allow good impregnation by aliquid resin; this layer is bordered by a winding of the stripsimpregnated with a resin and loaded with glass fibres. The presence ofthe adherent filaments in a material such as Dacron® means that inoperation the covering cannot withstand temperatures of 200° C. orhigher.

Document U.S. Pat. No. 4,666,318 discloses a self-lubricating coveringintended for very specific applications in the aeronautical field (lowpressure and low amplitudes), formed by a plastic material containingPTFE, co-operating with an opposing part having a roughness which is notgreater than 0.050 microns CLA and a hardness of not less than 1000 VPN.

It is understood that the determination of a friction covering combininga low coefficient of friction, a good mechanical performance (inparticular a good tearing strength), and the ability to retain goodfriction and mechanical properties up to operating temperaturescomprised between 250° C. and 300° C. (or even up to 320° C. intransient operation) means being able, under industrially acceptableconditions and at a reasonable cost, to combine a resin retaining a goodmechanical performance above 250° C. while exhibiting a satisfactoryadherence, including above this temperature threshold, with liningelements having a particularly low coefficient, therefore a priori notvery adherent to this resin.

An object of the invention is to meet this need.

To this end, the invention proposes a self-lubricating compositefriction part being able to be subjected in operation to temperatures atleast equal to 250° C., comprising, along the friction surface, a singlelayer of a fabric formed by weft and warp strands ofpolytetrafluoroethylene, this fabric being impregnated with aheat-stable resin having a glass transition temperature at least equalto 250° C.

It may be noted that, unlike the solutions already proposed, theinvention teaches the utilization of a single layer of a fabric, thewarp strands and the weft strands of which are all made ofpolytetrafluoroethylene (PTFE). Thus, the invention recommendsincreasing the section of the strands constituting the fabric (where themore recent known solutions tended to envisage several thin layers), soas to promote a good anchoring by the resin and to only use strands ofPTFE where the more recent known solutions tended to combine in one andthe same fabric, strands of PTFE with different strands, having a betteradherence with the resin. In fact, it appears that the fact ofutilizing, in the layer protecting the surface of the part, only asingle layer of fabric of PTFE has the advantage of increasing thetearing strength, given the continuity of the strands throughout thethickness of this protective layer, while promoting a good anchoring ofthis layer in the resin owing to the spaces remaining between thestrands of the fabric.

It can be said that such a fabric is self-lubricating.

According to advantageous characteristics of the invention:

-   -   The fabric is a weave formed by crossings of pairs of weft        strands and of pairs of warp strands; it can in particular be a        2/2 twill,    -   The weft strands or the warp strands are formed by short fibres        linked to one another,    -   The fabric has a thickness of at least 0.10 mm, advantageously        at least 0.30 mm, preferentially at least 0.50 mm,    -   The weft strands and the warp strands have a count of at least        100 dtex, preferentially at least 400 dtex,    -   The resin is a thermosetting polyimide,    -   The part also comprises a reinforcing layer bordering the fabric        opposite the friction surface, this reinforcing layer being        impregnated with the same resin as the fabric,    -   The part constitutes a bearing or a guide rail, among various        possible applications.

The product of the invention appears to have the advantage of beingself-lubricating in operation, at temperatures greater than 250° C.,which can range up to 300° C. continuously, or even peak at 320° C.,while having a very low coefficient of friction (comprised between 0.01and 0.2) equivalent to that of untreated PTFE (without additive orwithout lining) but resistant to loads greater than 40 N/mm².

By analogy, the invention proposes a method for the manufacture of aself-lubricating composite friction part of the abovementioned typeaccording to which a layer of fabric is formed by the helical winding ofa strip of fabric formed by weft strands and warp strands, allconstituted by polytetrafluoroethylene, on a mandrel according to awinding angle such that the strip comes edge-to-edge with itself aftereach turn, the fabric is impregnated with a heat-stable resin having aglass transition temperature of at least 250° C. This resin isadvantageously a thermosetting polyimide.

Objects, characteristics and advantages of the invention will becomeapparent from the description which follows, given by way ofnon-limitative illustrative example, with reference to the attacheddrawings in which:

FIG. 1 is a perspective view of a friction part according to theinvention,

FIG. 2 is a view of a preferred example of weaving the friction layer ofthis friction part,

FIG. 3 is a cross-sectional view of this friction layer bordered by areinforcing layer, and

FIG. 4 is a simplified diagram of the method of formation of a frictionpart such as that of FIGS. 1 to 3.

A friction part according to the invention essentially comprises afriction layer having a free surface S intended to be opposite anopposing part; advantageously, this friction part also comprises areinforcing layer bordering the friction layer opposite the frictionsurface in order to strengthen the mechanical performance of this layer.

In the example of FIG. 1, the friction part is a bearing 1 intended toreceive, in its longitudinal bore 1A, a shaft (not shown). In a variant,it can also be a slide receiving a rod in translational motion. Thefriction layer which borders the friction surface (therefore theinternal surface) is denoted by the reference 2, while the reinforcinglayer is denoted by the reference 3. This layer 3 here has a thicknesssubstantially greater than that of the friction layer; in fact, thefriction layer in practice has a thickness of the order at most a fewmillimetres (no more than 3 mm in practice) while the reinforcing layercan be several millimetres, or even a few centimetres in thickness,depending on requirements. However, it goes without saying that thereinforcing layer, when it exists, can have any relative thickness withrespect to the friction layer.

The function of the friction layer is to guide, with the least possiblefriction, the opposing part which is the abovementioned shaft whileretaining its physical integrity in operation, for as long as possible,including at operating temperatures of at least 250° C. continuously,and peaking above 300° C. (for example up to of the order of 320° C.).

In order to do this, the friction layer consists of a single layer of afabric of strands made of coated polytetrafluoroethylene (or PTFE) in amatrix formed by a heat-stable resin having a glass transitiontemperature greater than the maximum continuous operating temperature,therefore of at least 250° C., or even as close as possible to 300° C.

The concept of polytetrafluoroethylene or PTFE denotes here the variousforms of this compound, including the expanded version known as “ePTFE”.

The weave of the fabric, i.e. the relative configuration of the strandsconstituting this fabric, is selected so as to form, between the variousstrands, flow channels that can be filled with the heat-stable resin. Infact it is understood that, as the PTFE has practically no adherencewith the other materials, the anchoring of the fabric in the matrix canonly be done by entangling the irregular filaments constituted by theresin filling the various flow channels existing through the fabric,which filaments are connected along the strands of PTFE close to thefriction surface.

It is understood that a compromise, depending on the applications, is tobe found regarding the section and the number of the resin flow channelsthrough the fabric; the more numerous and the wider these flow channels,the better the anchoring of the strands in the PTFE, but the lower thefraction of the friction surface which is formed by strands of PTFE.Conversely, the greater the fraction of friction surface formed ofstrands in PTFE, the better the friction behaviour of the friction part,but the weaker the anchoring of the fabric in the matrix.

It appears desirable that there is a flow channel for the resin at eachcrossing of the weft strands and the warp strands.

Among the common weaves, it appears that a twill, and more precisely a2/2 twill formed by the interweaving of pairs of weft strands and pairsof warp strands, allows the constitution, between the strands of anetwork, of channels filled with resin which is sufficiently dense toensure a good anchoring of the fabric despite the absence of adherencebetween the strands and the resin, while giving the opposing part asignificant surface formed by PTFE.

Such a 2/2 twill is shown in FIGS. 2 and 3, where weft strands aredenoted by the reference 5 and warp strands are denoted by the reference6, leaving the interstices 7 free for the resin to pass through, subjectto the weaving not being too tight. Good results have been obtained witha 2/2 twill.

Yet more advantageously, the weft and warp strands are each formed by asingle filament formed by PTFE fibres linked to each other by twisting,which means that the filaments, and therefore the weft and warp strands,have an irregular surface which contributes to the good anchoring of thefabric in the matrix.

Good results have been obtained with such a weaving of monofilamentsformed by fibres with an average diameter of 0.1 to 0.14 mm.

These fibres preferably have a count of more than 400 dtex;advantageously at least 750 dtex; very satisfactory tests have beenobtained with fibres of 833 dtex.

In a variant, the fabric is formed by strands each formed by severalfilaments, continuous or formed by short fibres as in the abovementionedexample; in such a case, the count of the filaments can be lower, forexample of the order of 350 to 450 dtex for bifilament strands, or evenless. The weaving can be done by assembling strands of two or threepieces, with or without twisting.

According to yet another variant, the surface of the filaments isdeliberately rendered irregular, for example by the formation ofmicro-notches.

The thickness of the fabric is at least 0.30 mm, or even at least 0.5mm; values ranging beyond one millimetre can be envisaged; this makes itpossible to determine the section of the strands to be used. The weftstrands and the warp strands are advantageously identical. Their sectionis, in the example considered in FIGS. 2 and 3, that of a disc. In avariant which is not shown, this section is rectangular, with forexample a form factor (ratio between the largest dimension and thesmallest dimension) which is preferably at least 2.

The heat-stable resin is advantageously selected from the thermosettingpolyimides, the resins based on cyanate ester or from thepolyetherketones (polyetheretherketone—PEEK, orpolyetherketoneketone—PEKK, in particular). These resins have at minimumglass transition temperatures greater than 280° C. Among thethermosetting polyimides, the polybismaleimides—or BMI—can be mentioned.

It does not appear to be useful to incorporate a filler into theheat-stable resin.

As the reinforcing layer, when it exists, has the function of forcingthe friction layer to retain its shape despite the pressure appliedbetween the friction part and the opposing part, including at hightemperature, it is understood that it is advantageous for thisreinforcing layer to be constituted by a material having a very lowthermal expansion coefficient, typically at most equal to 13.10⁻⁶ K⁻¹(corresponding to steel); it is within the scope of a person skilled inthe art to define the geometry and the constitution of this reinforcinglayer as a function of requirements. It can in particular comprisestrands or fibres of carbon, glass or aramide, free or combined in afabric (sometimes referred to as roving).

The fabric of the friction layer is advantageously available as a strip,which confers a great freedom for shaping the friction layer by using,if necessary, a preform the profile of which is the negative of the formof the friction surface to be obtained. The width of the strip can beselected as a function of requirements; it is advantageously selectedbetween 5 mm and 2 m, for example between 1 cm and 10 cm, preferablybetween 1.5 cm and 3 cm.

In the abovementioned case where the friction part is a bearing, itsmanufacture can start by winding such a strip of fabric around a mandrelthe external diameter of which is equal to the internal diameter of thebearing to be produced; the strip is wound helically so as to ensure anedge-to-edge contact of the successive turns formed on the mandrel (seeFIG. 4). It is understood that the width of the strip 10 determines theinclination of the weft strands and the warp strands with respect to theaxis of the mandrel and therefore of the future bearing. In fact, theweft strands and the warp strands are arranged longitudinally andtransversally to the strip of fabric, respectively.

It is understood that a continuity of both the weft strands and warpstrands in the friction layer contributes to the preservation of a goodintegrity during the operating life of the bearing; for these strands,an inclination of 40° to 60° with respect to the longitudinal axis ofthe bearing appears to be favourable for this. In the case of anedge-to-edge winding and furthermore for a fabric (and not for astrand), it is preferable to have a winding following an angle comprisedbetween 65° and 89°.

The reinforcing layer can be formed by winding a strand of carbon orglass, or any other appropriate material with what can be, apparently,any angle of inclination. In the case of windings of strands, theoptimum angle appears to be comprised between 40° and 60°, but thisangle can vary depending on the applications and the desired mechanicalcharacteristics.

The strip of fabric on the one hand, and the strand of the reinforcinglayer on the other hand, are advantageously impregnated beforehand witha heat-stable resin; however it is understood that, if the reinforcinglayer is formed using the same resin as for the friction layer, asubsequent heat treatment can help to firmly attach the matrices to oneanother.

Instead of being formed by a winding of a strip, as a variant the PTFEfabric is constituted by a braided tubular sleeve.

It is easily understood that, for a friction part of the slide type, thefriction layer can be simply formed by attaching the abovementionedfabric to an underlying reinforcing layer.

By way of example, a friction bearing was formed in the followingmanner.

A PTFE fabric with a thickness of 0.3 mm was selected with a 2/2 twillweave, in the form of a strip with a width of 3 cm. This strip wasimmersed in an impregnation bath, maintained at 110° C., containing aheat-stable resin of polybismaleimide (BMI) type having a glasstransition temperature of 285° C.

This impregnated strip was wound onto a mandrel, taking care to coverthe entire surface of the mandrel without any overlapping between thesuccessive turns, i.e. so as to form a single continuous layer over theentire surface of the mandrel which was intended to form a bearing (or aplurality of bearings). Advantageously, the mandrel was itself alsomaintained at the temperature of the impregnation bath.

A strand of epoxy glass impregnated beforehand with the same resin(referred to as glass filament of the roving type) was then wound.

The polymerization cycle followed comprised treatment for 4 hours (alonger duration is possible) at 170° C., demoulding, and an additionalcuring treatment for 4 hours (a longer duration is possible) at atemperature comprised between 230° C. and 250° C.

It has been noted that it was difficult to cut the PTFE fibres duringsubsequent machining, which confirms the good performance with regard towear of the assembly.

Tribological tests were carried out under the following conditions:

-   -   Oscillation of the axis of amplitude: 100°.    -   Projected pressure: 80 MPa.    -   Average velocity: 8 mm/s    -   Average PV (pressure×velocity): 0.64 MPa·m/s.    -   Initial shaft/bearing clearance: between 0.1 and 0.2 mm.    -   Dimensions of bearing: Øint 30×Øext 36×Lg 20.    -   Initial greasing: None    -   Opposing shaft of the solutions tested: 16 NC 6 case-hardened    -   Maximum duration of test: 1 month (350,000 cycles)    -   Ambient temperature

While an increase in the coefficient of friction was noted with a knownbearing (formed from a polyester fabric coated with a resin loaded withPTFE, or from a fabric formed by strands of polyester and strands ofPTFE with less than 50% maximum of PTFE), up to 0.04, or even 0.08, thecoefficient of friction for the bearing of the invention appears toremain substantially constant at a value of barely 0.02 up to 350,000cycles.

When monitoring the change in temperature at the centre of the shaft ofthe opposing part, it is noted that this temperature increased up tonearly 50° C., or even 60° C. with the known bearing, the temperatureremained below 40° C. with a bearing according to the invention; thisclearly reflects that the energy to be dissipated with a bearingaccording to the invention is less than that with a known bearing.

Even so, it is noted that the general wear of the bearing according tothe invention is greater than those of the known bearing, while theopposing part shows very little wear; it can however be assumed thatthis wear is only apparent, reflecting in fact the existence of aphenomenon of crushing of the friction layer under the pressure of thecontact applied.

The bearing according to the invention was moreover tested under thefollowing conditions:

-   -   Oscillation of the axis of amplitude: 100°.    -   Projected pressure: 80 MPa.    -   Average velocity: 8 mm/s    -   Average PV (pressure×velocity): 0.64 MPa·m/s.    -   Initial shaft/bearing clearance: between 0.1 and 0.2 mm.    -   Dimensions of bearing: (Øint 30×Øext 36×Lg 20.    -   Initial greasing: None    -   Opposing shaft of the solutions tested: 16 NC 6 case-hardened    -   Maximum duration of test: 1 month (350,000 cycles)    -   Temperature varying from 50° C. to 280° C. (surroundings)—it was        difficult to maintain the temperature constant during these        temperature levels

The coefficient of friction appears to remain substantially constantdespite the increases to 280° C.

These tests establish that the bearing according to the inventionsatisfactorily combines a very low temperature coefficient and a goodperformance at a temperature up to above 250° C., in the range 250°C.-280° C.

The invention claimed is:
 1. A self-lubricating composite friction partconfigured for operation temperatures at least equal to 250° C.,comprising: along a friction surface, a friction layer consisting of asingle layer of a woven layer fabric formed of a 2×2 twill having pairsof crossing weft and warp strands all consisting ofpolytetrafluoroethylene, said fabric being impregnated with aheat-stable resin having a glass transition temperature equal to orgreater than 250° C.
 2. The self-lubricating composite friction partaccording to claim 1, wherein the weft strands or the warp strands areformed by short fibres linked to one another.
 3. The self-lubricatingcomposite friction part according to claim 1, wherein the fabric has athickness of at least 0.10 mm.
 4. The self-lubricating compositefriction part according to claim 3, wherein the fabric has a thicknessof at least 0.5 mm.
 5. The self-lubricating composite friction partaccording to claim 1, wherein the weft strands and the warp strands havea count of at least 100 dtex.
 6. The self-lubricating composite frictionpart according to claim 1, wherein the resin is a thermosettingpolyimide.
 7. The self-lubricating composite friction part according toclaim 1, further comprising: a reinforcing layer bordering the fabricopposite the friction surface, said reinforcing layer being impregnatedwith a same resin as the fabric.
 8. The self-lubricating compositefriction part according to claim 1, constituting a bearing.
 9. Theself-lubricating composite friction part according to claim 1,constituting a guide rail.
 10. A method Moth d for the manufacture of aself-lubricating composite friction part according to claim 1,comprising: forming a friction layer consisting of a single layer offabric formed by the helical winding of a strip of fabric formed by weftstrands and warp strands, all constituted by polytetrafluoroethylene, ona mandrel according to a winding angle such that the strip comesedge-to-edge with itself after each turn; and impregnating the fabricwith a heat-stable resin having a glass transition temperature of atleast 250° C.
 11. The method according to claim 10, wherein the resin isa thermosetting polyimide.
 12. The self-lubricating composite frictionpart according to claim 2, the fabric has a thickness of at least 0.10mm.
 13. The self-lubricating composite friction part according to claim2, wherein the weft strands and the warp strands have a count of atleast 100 dtex.
 14. The self-lubricating composite friction partaccording to claim 3, wherein the weft strands and the warp strands havea count of at least 100 dtex.
 15. The self-lubricating compositefriction part according to claim 4, wherein the weft strands and thewarp strands have a count of at least 100 dtex.
 16. The part accordingto claim 1, wherein the weft and warp strands are each formed by asingle filament formed by polytetrafluoroethylene fibres linked to eachother by twisting so that the weft and warp strands have irregularsurfaces.
 17. A self-lubricating composite friction part configured foroperation temperatures at least equal to 250° C., comprising: along afriction surface, a friction layer consisting of a single layer of awoven layer fabric formed of weft and warp strands all consisting ofpolytetrafluoroethylene, said fabric being impregnated with aheat-stable resin having a glass transition temperature equal to orgreater than 250° C., wherein the fabric is a weave formed by crossingsof pairs of weft strands and of pairs of warp strands, and wherein theweft and warp strands are each formed by a single filament formed bypolytetrafluoroethylene fibres linked to each other by twisting so thatthe weft and warp strands have irregular surfaces.